The Americans with Disabilities Act (ADA), which was signed into law as §12181 of Title 42 of the United States Code on Jul. 26, 1990, is a wide-ranging legislation intended to make American society more accessible to people with disabilities. The law, which took effect on Jul. 26, 1992, mandates, among other things, standards for access to public facilities, including public sidewalks. The law not only requires that curb cuts be made at intersections and crosswalks to facilitate wheelchair access, but also mandates specifications for slopes and transitions between two surfaces of different levels. Some of the relevant provisions of the law are as follows:
4.5.2 Changes in Level. Changes in level up to ¼ inch (6 mm) may be vertical and without edge treatment. Changes in level between ¼ inch and ½ inch (6 mm and 13 mm) shall be beveled with a slope no greater than 1:2. Changes in level greater than ½ inch (13 mm) shall be accomplished by means of a ramp that complies with 4.7 or 4.8.
4.7.2 Slope. Slopes of curb ramps shall comply with 4.8.2. Transitions from ramps to walks, gutters, or streets shall be flush and free of abrupt changes. Maximum slopes of adjoining gutters, road surface immediately adjacent to the curb ramp, or accessible route shall not exceed 1:20.
4.8.1 General. Any part of an accessible route with a slope greater than 1:20 shall be considered a ramp and shall comply with 4.8.
4.8.2 Slope and Rise. The least possible slope shall be used for any ramp. The maximum slope of a ramp in new construction shall be 1:12. The maximum rise for any run shall be 30 inches (760 mm). Curb ramps and ramps to be constructed on existing sites or in existing building or facilities may have slopes and rises as allowed in 4.1.6(3)(a) if space limitations prohibit the use of a 1:12 slope or less.                4.1.6(3)(a)1. A slope between 1:10 and 1:12 is allowed for a maximum rise of 6 inches.        4.1.6(3)(a)2. A slope between 1:8 and 1:10 is allowed for a maximum rise of 3 inches. A slope steeper than 1:8 is not allowed.        
Historically, trip hazards caused by uneven lifting and settling of contiguous sidewalk sections have been eliminated either by tearing out the old concrete and replacing it with new slabs having no abrupt transitions between joints, by forming a transition ramp on the lowermost section with macadam, or by creating a chamfer on the edge of the uppermost section. The first method represents the most expensive fix. The second method, which uses dark-colored macadam on a light-colored sidewalk, is unsightly. If the chamfer is made using a surface cutter or grinder, the second method is slow, given that all material removed through grinding must be pulverized. In addition, if the process is performed with a drum cutter, the equipment is relatively expensive and leaves a rough surface. In addition, most equipment used heretofore is incapable of removing the trip hazard over the entire width of a sidewalk. Furthermore, if two adjacent sidewalk slabs have twisted in opposite directions as they have settled or raised, it may be necessary to create a ramp across a portion of the width of the sidewalk on both sides of the joint.
U.S. Pat. No. 7,402,095 to M. Ballard Gardner (Gardner '095), dated Jul. 22, 2008, provides a method and apparatus that reduces the time and cost required to form chamfers on uneven, contiguous sidewalk slabs. The method and apparatus are capable of removing a trip hazard over the entire width of a sidewalk, and of chamfering portions of two intersecting slabs at a common joint. The method of Gardner '095 employs a right-angle grinder motor, to which a circular, diamond-edged cutting blade has been flush mounted, to slice off the trip hazard at an angle that complies with the ADA.
Because the time, effort and cost required to remove a particular trip hazard depend on the amount of concrete that must be cut, it is important to accurately estimate a job beforehand so that the party who will perform the work can accurately estimate the time required to complete the job, and provide an accurate cost estimate to the paying party. As the time required to perform any chamfer cut is directly related to the surface area of the cut, it is essential to calculate the surface area for each chamfer cut to be made during the estimating process. The surface area of a chamfer cut is a function of both the average thickness, at the adjoining edge of the slab, of the concrete to be removed, and the maximum slope permitted by the ADA under §4.1.6(3)(a)1 and §4.1.6(3)(a)2. For the sake of transparency in the estimating process, each proposed cut must be identified by location and projected cost.
In order to accurately estimate the surface area of a chamfer cut, the estimator must calculate the width and average length of each cut. As the average length of a cut is typically a function of the average difference in height between two adjoining concrete slabs, it is imperative that the estimator determine the average difference in height for each cut. Such a calculation has, heretofore, been performed by an estimator kneeling at the site of a potential cut and measuring the average height of the trip hazard with a straight edge and a tape measure. The straight edge is used to project the surface of the more elevated slab over the edge of the lower-lying slab, where the measurement is made. Such a procedure is time consuming, and requires the estimator to kneel at the location of each proposed cut.
What is needed is a gauge that will enable an estimator to measure variations in the levels of two adjacent concrete slabs while standing. In addition, the estimator will also be able to record the measurement data, as well as the location, for each proposed cut in a convenient manner so that calculations may be later made and transformed into a quote for the customer.